A display backplane is an important component in a display device, such as a liquid crystal display (LCD), organic light emitting diode display (OLED), or other display technology. The display backplane includes a substrate that functions as a platform upon which circuitry is formed to cause a display screen to display images. Typically, the backplane of such a display includes an array of pixel transistors that provide an electrical signal to an array of pixel elements, such as OLED cells, to produce light that results in an image to be viewed. Additional circuitry for the display includes row and column drivers, which are not typically located on the backplane. The row and column drivers decode the incoming video data to individually activate the pixel transistors and thereby individually control the pixels.
Because the pixel transistors are typically located on the backplane itself, the pixel transistors are formed as thin film transistors when used in a thin display such as for thin screen computer and television monitors, telephones, and other compact devices. Because the row and column drivers are not typically located on the backplane, they are not necessarily thin film transistors. However, the row and column drivers occupy separate space, such as on an integrated circuit chip installed on a display circuit board.
The interconnections between the row and column drivers and the backplane array can be complex. As the number of rows and columns increase, the interconnect density increases. Even when the row and column drivers are silicon chips bonded to the glass, the level of interconnect complexity can become prohibitive.
It is desirable in some display screen applications to eliminate or dedicate for other purposes the space required for the row and column driver chip and/or to bring the row and column drivers into closer proximity with the pixel transistors. Therefore, it is desirable to move the row and column drivers directly onto the backplane along with the pixel transistors. However, because the row and column drivers must have very fast switching capabilities, conventional thin film transistor constructions utilizing low mobility semiconductor channels such as amorphous silicon become problematic.
It is advantageous, particularly for an organic light emitting diode-based display, to have thin film transistors including a semiconductor with as large an electron mobility as possible. In general, the electron mobility directly affects transistor speed and/or transistor size. Semiconductors like amorphous silicon can have field effect mobilities on the order of 0.5 cm2/V-sec. Materials such as polysilicon have higher mobilities (greater than 20 cm2/V-sec) but require higher processing temperatures and more complex fabrication procedures.